Miniature circuit breakers are commonly used for providing automatic circuit interruption upon detection of undesired overcurrent conditions on the circuit being monitored. These overcurrent conditions include, among others, overload conditions, ground faults and short-circuit conditions.
Miniature circuit breakers typically include an electrical contact mounted on a movable contact carrier which rotates away from a stationary contact in order to interrupt the current path. The contact carrier is pivotally mounted to a rotatable blade housing, and a spring is used to bias the movable contact toward the stationary contact during normal current conditions. The type of overcurrent condition dictates how quickly the contact carrier must rotate away from the stationary contact. For example, in response to overcurrent conditions at relatively low magnitudes but present for a long period of time, circuit breakers generally employ a tripping mechanism to rotate the blade housing carrying the contact carrier. Since the contact carrier rotates with the blade housing, the contact on the movable contact carrier is forced away from the stationary contact. In response to overcurrent conditions at relatively high magnitudes, circuit breakers must break (or blow-open) the current path very quickly, reacting much faster than the reaction time for the tripping mechanism. In this case, the contact carrier rotates to an open position prior to actuation of the tripping mechanism.
When the electrical contact on the movable contact carrier separates from the stationary contact in response to an overcurrent condition, undesired arc energy develops between the separating contacts because of their voltage differential. This arc energy may be characterized as a discharge of electricity through a gas, where the voltage differential between the separating contacts is approximately equal to the ionization potential of the gas. The arc energy is undesirable because it has a tendency to flow back or collapse back into the gap separating the contacts, thereby exposing the movable contact carrier and the stationary contact carrier to the arc energy. The movable contact carrier and stationary contact carrier may be eroded, melted, or vaporized when exposed to the arc energy without some sort of protective device. If one or both of the contact carriers are damaged to the extent that there is an excessive reduction in their cross-sectional area, the contact carriers could fail to properly interrupt the circuit in response to an overcurrent condition. The arc energy is also undesirable because it has a tendency to flow toward the tripping mechanism of the circuit breaker, where the arc energy can damage the components of the tripping mechanism. One component of the tripping mechanism which is susceptible to damage is the toggle spring, which is often detachably connected at one end to a hook on the movable contact carrier. Without some sort of protective device, the arc energy can fuse the toggle spring to the carrier hook or cause the toggle spring to anneal and thereby loose its elasticity.
Accordingly, there is a need for an arrangement for protecting a movable contact carrier, a stationary contact carrier, and a tripping mechanism of a miniature circuit breaker from arc energy generated during a circuit interruption.